In order to fully characterize the effect of various photolithography process steps it is important to monitor the effect of each of these process steps on the absorption of the photoresist. Photoresist contains photosensitizer such as napthaquinine diazide (NAQ1), whose chemical structure is shown in FIG. 1. Exposure of photoresist to light, typically of wavelength .lambda.=300-450 nm, causes the sensitizer to convert to an indene carboxylic acid (ICA), which is soluble in an aqueous base developer.
This arrangement can be monitored using light in the ultraviolet, visible or infrared wavelengths. In the ultraviolet, the change in absorption is due to electronic excitations. The long wavelength electronic excitations are associated with the .pi. or double bond electron clouds indicated to FIG. 1. The excitation wavelength is controlled by many factors, with the length of the .pi. bond being one of the most significant; the longer the .pi. bond, the lower is the energy required for electronic excitation, and hence the longer is the absorption wavelength.
The bond lengths in NAQ1 are as large as or larger than the corresponding bond lengths in ICA; hence the NAQ1 has longer wavelength absorption than the ICA. This is shown in the comparison of transmission coefficients in the UV and low visible wavelength regions for three different photoresist materials, AZ5214, A4110 and AZ1370 in FIGS. 2, 3 and 4, respectively. Transmission coefficient differences between exposed and unexposed photoresist material are significant between .lambda..apprxeq.300 nm and .lambda..apprxeq.450 nm for each material. The photoresist becomes more transparent upon exposure at the usual exposure wavelengths of .lambda.=300-450 nm. A theoretical equation relating transmission coefficient before exposure (T.sub.o) and after exposure (T) to a dose D (Joules/cm.sup.2) of light at a prescribed irradiation wavelength .lambda. is EQU T=[1+(1/T.sub.o -1)e.sup.-CD ].sup.-1
where the constant C.apprxeq.0.012-0.015 cm.sup.2 /Joule and will vary with .lambda. and with the photoresist material. As a result, the change in visible/uv absorption of the photoresist material can be used to determine the degree of exposure.
The reaction or conversion of the photoresist component can also be monitored in the infrared wavelengths. The infrared radiation is absorbed by bond vibrations. The --N.dbd.N bond of the diazo side group has a vibration at 2100 cm.sup.-1, isolated from all other bond vibrations. As the NAQ1 is exposed, the absorption due to --N.dbd.N vibrations is reduced in substantially the same proportion as the number of --N.dbd.N bonds is reduced by conversion.